Recovery of high quality natural gas is becoming increasingly important as the price of energy continues to rise. Furthermore, the use of natural gas tends to lessen the guantity of pollutants produced for a given amount of energy generated when compared to certain other commonly used means of energy generation.
One problem often encountered in nautral gas recovery whether from natural gas wells or petroleum reservoirs is nitrogen contamination. Natural gases which contain significant amounts of nitrogen may not meet minimum heating value specifications, reduce pipeline capacities and require additional compression horsepower and fuel consumption. Nitrogen removal from natural gases has therefore attained increased importance.
In many cases, successful recovery of petroleum or natural gas requires the use of an enhanced recovery technique. One such often used technique involves the injection into the reservoir of a fluid which will not support combustion; an often used fluid for this technique is nitrogen or a nitrogen-containing gas due to its relatively low cost compared to argon, helium and the like. However, the use of this technique increases the level of nitrogen contaminant in the gas recovered from the reservoir, i.e., the natural gases, above their naturally-occurring nitrogen concentration.
Nitrogen injection for enhanced oil or gas recovery introduces a further problem because the nitrogen concentration in the natural gases does not remain constant over the life of the recovery operation. Although the nitrogen concentration variation will strongly depend upon particular reservoir characteristics, a general pattern is predictable. Typically during the first few years that enhanced recovery with nitrogen injection is employed, the nitrogen concentration in the natural gases may remain at about the naturally-occurring level, increasing thereafter, for example, by about 5 percentage points after 4 years, by about 15 percentage points after 8 years, by about 25 percentage points after 10 years and by about 50 percentage points after 16 years.
The problem of a changing nitrogen concentration in natural gases recovered from the reservoir further complicates the economics of recovery. As shown, for example, in "Design Considerations For Nitrogen Rejection Plants," R. A. Harris, Apr. 17, 1980, The Randall Corp., Houston, Tex., the specific nitrogen removal process employed will be dictated by the nitrogen concentration. A nitrogen concentration of from 15 to 25 percent requires one type of process, a nitrogen concentration of from 25 to 40 percent requires another, a nitrogen concentration of 40 to 50 percent still another process, and a concentration greater than about 50 percent yet another process. The alternative, i.e., the use of only one process as the nitrogen concentration in the natural gases varies, is believed to result in severe operating inefficiencies.
In response to the problem of nitrogen contamination of natural gases, several methods of separating the nitrogen from the natural gases have been developed. One known method employs a dual pressure double distillation column; this type of arrangement is often used in the fractionation of air into oxygen and nitrogen. However, this method is generally limited to applications where the nitrogen concentration of natural gases is greater than about 25 percent. Where the nitrogen concentration is lower than 25 percent, the quantity of reflux liquid that can be generated in the high pressure column when using the conventional double column process decreases to the extent that proper fractionation cannot be conducted in the low pressure column.
A description of a typical double distillation column process for separating nitrogen from natural gas is disclosed in Jones, "Upgrade Low-Btu Gas," Hydrocarbon Processing, September 1973, pp. 193-195. Reflux for the low pressure column is provided by a nitrogen liquid generated within the high pressure column. At low nitrogen feed gas concentrations the required liquid nitrogen reflux cannot be generated resulting in high methane losses in the nitrogen exit stream.
Those skilled in the art have addressed this problem by recycling a portion of the nitrogen exit stream back to the natural gas feed stream, thus keeping the nitrogen concentration high enough for effective separation in the double distillation column. This method, however, is disadvantageous from two standpoints. First, use of a nitrogen recycle in this manner increases the plant size requirements. Second, this process leads to significantly increased power requirements since relatively pure nitrogen from the exit stream must be separated all over again from the natural gas feed.
Also known are single column processes for removing nitrogen from natural gas. One such process is disclosed in U.S. Pat. No. 2,583,090--Cost, wherein a high pressure feed having a nitrogen concentration of about 40 percent is cooled and expanded into a single fractionation column. Reflux liquid is obtained by condensing overhead nitrogen gas in a liquefier by heat exchange with work expanded nitrogen gas. At lower nitrogen feed gas concentrations, for example at about 30 percent nitrogen, a nitrogen recycle stream is employed to develop the additional refrigeration and reflux required. This is accomplished by warming some of the work expanded nitrogen gas, compressing it to about the fractionation pressure, cooling it against the nitrogen gas to be compressed and then mixing it with the nitrogen gas which is to be work expanded. This process is relatively expensive from both a capital equipment cost and a power consumption cost standpoint.
Another single column process to remove nitrogen from methane is disclosed in U.S. Pat. No. 2,696,088--Toomey. Reflux for the fractionation column which is operated at relatively low pressure, is provided by liquefying a portion of the nitrogen overhead. The necessary refrigeration for this liquefaction is provided by a cascaded refrigeration system employing an ammonia cycle, an ethylene cycle and methane cycle. This process is disadvantageous because it is considerably complex and consumes a large amount of power.
A process which can effectively separate nitrogen from natural gases wherein the nitrogen concentration of the natural gas feed is initially low, and which avoids the heretofore disclosed uneconomical methods required to compensate for the low nitrogen concentration in the feed would be highly desirable.
More importantly, none of the known processes for removing nitrogen from natural gases is directed to situations where the nitrogen concentration in the feed gas increases substantially over time such as is typically experienced when nitrogen injection enhanced recovery is employed. Processes which adequately separate nitrogen from natural gases at high nitrogen feed gas concentrations must be significantly altered to achieve good separation at low nitrogen feed gas concentrations. These alterations invariably increase the capital and/or operating costs of the system in order to achieve the desired separation. Therefore, a process which will achieve good separation of nitrogen from natural gases over a wide range of nitrogen concentrations in the feed, while substantially avoiding the increased capital and/or operating costs of heretofore available processes is highly desirable.
Therefore, it is an object of this invention to provide an improved process for the separation of nitrogen from natural gases.
It is another object of this invention to provide an improved process for the separation of nitrogen from natural gases capable of handling a natural gas feed stream in which the nitrogen concentration is relatively low.
It is a further object of this invention to provide an improved process for the separation of nitrogen from natural gases capable of handling a natural gas feed stream in which the nitrogen concentration may vary considerably.